1. The Field of the Invention
This invention relates to semiconductor processing technology and, more particularly, to novel systems and methods for heating fluids and making heaters carrying ultra-pure fluids for processing operations.
2. The Background Art
The semiconductor manufacturing industry relies on numerous processes. Many of these processes require transportation and heating of de-ionized (DI) water, acids and other chemicals. By clean or ultra-pure is meant that gases or liquids cannot leach into, enter, or leave a conduit system to produce contaminants above permissible levels. Whereas other industries may require purities on the order of parts-per-million, the semiconductor industry may require purities on the order of parts-per-trillion.
Chemically clean environments maintained for handling pure de-ionized (DI) water, acids, chemicals, and the like, must be maintained free from contamination. Contamination in a process fluid may destroy hundreds of thousands of dollars in value by introducing contaminants into a process during a single batch. Several difficulties exist in current systems for heating, pumping, and carrying process fluids (e.g., acids, DI water, etc.). Leakage into or out of a liquid must be eliminated. Moreover, leaching and chemical reaction between any contained fluid and the carrying conduits must be eliminated.
Elevated temperatures in semiconductor processing are often over 100xc2x0 C., and often sustainable over 120xc2x0 C. In certain instances, temperatures as high as 180xc2x0 C. may be approached. It is preferred that all heating and carrying of process fluids include virtually no possibility of contact with any metals regardless of the ostensibly non-reactive natures of such metals, regardless of a catastrophic failure of any element of a heating, transfer, or conduit system.
Conventional immersion heaters place a heating element, typically sheathed in a coating, directly into the process fluid. The heating element and process fluid are then contained within a conduit. Temperature transients in immersion heaters may overheat a sheath up to a melting (failure) point. A failure of a sheath may directly result in metallic or other contamination of the process fluid. Meanwhile, temperature transients in radiant heaters may fracture a rigid conduit.
A heating alternative is needed that does not have the risks associated with conventional radiant and immersion-heating elements. A system is needed that is both durable and responsive for heating process fluids. Failure that may result in fluid contamination is an unacceptable risk.
In view of the foregoing, it is a primary object of the present invention to provide a heater for handling process fluids at elevated temperatures in the range of 0xc2x0 C. to 180xc2x0 C. It is an object of the invention to provide a heater having electrical resistance in close proximity to a process fluid for heating by conduction and convection without exposing process fluids to a prospect of contamination, even if electrical failures or melting of conductive paths should occur within a heater.
Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a method and apparatus are disclosed in one embodiment of the present invention as including a heater comprising one or more tubes of quartz. Tubes may be abutted end-to-end with an adaptor (e.g., fluorocarbon fitting) fitted to transition between two tubes in a series. One pass or passage, comprising one or more tubes of quartz in a series, may be fitted on each end to a manifold (e.g., header/footer) comprised of a fluorocarbon material properly sealed for passing liquid into and out of the individual passage.
Individual tubes or conduits may improve the temperature distribution therein by altering the internal boundary layer of heated fluids passing therethrough. In one embodiment, a baffle tube, within the outer tube, may have a plug serving to center the baffle in the heating tube. The plug may restrict flow, such that the fluid inside the baffle does not change dramatically. Thus an annular flow between the baffle tube and the outer heating tube may maintain a high Reynolds number in the flow, enhancing the Nusselt number, heat transfer coefficient and so forth. Moreover, the temperature distribution may be rendered nearer to a constant value across the annulus, rather than running with a cold, laminar core.
In one embodiment, a heater may be manufactured by electroless nickel plating on a roughened (textured) surface. A resistive, conductive layer may extend along most of the length of a rigid (e.g., quartz) tube. The resistive coating may be configured to connect in series or to multi-phase power along the length of a single tube. Accordingly, a quartz tube may be roughened, etched, dipped, coated, and protectively coated. The quartz tube need not be heated to sinter the conductive layer, which may be plated as a continuous ribbon of well-adhered, resistive, conducting, metallic material.
The electrical length of the heated portion may be adjusted by application of an end coating for distributing current around a conduit tube. Conductive material and mechanical fasteners may be added to provide electrical connections between the end coating and power delivery lines. For example, braided cables or straps may be clamped around a soft, conductive interface material surrounding each end of a plated section of a conduit. Mechanical clamps may maintain normal forces against the surface, while accommodating expansion with temperature, without harming mechanical bonds between the conductive/resistive coating and the conduit (substrate).